This invention relates generally to data storage and retrieval, and more particularly to an improved method and apparatus for storing and retrieving large amounts of data contained in magnetic tape cartridges.
There exists a need in the marketplace, since the advent of computers such as the IBM 360/370, to store large amounts of data (e.g., in excess of 1 trillion bits) while consuming as little floor space as possible, and at the same time making the data readily available. Basically, two choices for data storage have existed: (1) online, usually direct access storage devices (DASD), which provides fast initial service time through sequential or direct processing; or (2) offline, usually manual 9-or 18-track magnetic tapes, which provides a low unit cost per unit storage. There exists a need, however, for many new applications were costly DASD are not justified but where manual tape systems are considered too slow or too inconsistent.
One early approach, used in the IBM 3850 Mass Storage System, consists of an array of data cartridges about 1.9 inches in diameter and 3.5 inches long, with a capacity of 50 million characters each. Each cartridge contains magnetic tape 2.7 inches wide and 64 feet long, on which data is organized in cylinders analogous to those of a disk file and can be transferred to the disk file one cylinder at a time. Up to 4700 cartridges can be stored in hexagonal compartments in a honeycomb-like apparatus that includes mechanisms for fetching cartridges from the compartments, for the reading and writing of data on them, and for the replacement of cartridges in the compartments. The fetching and replacing mechanism, however, is adapted to store and retrieve the cartridges from a linear rack. Accordingly, if a desired cartridge is located remotely from the retrieving mechanism at one end of the rack, the retrieving mechanism would be required to move to the end, retrieve the cartridge, and load it upon a drive. It is, therefore, apparent that if a subsequently selected cartridge is located at the other end of the rack, much wasted time would be utilized in the movement of the retriever mechanism between opposite ends of the rack.
Various other tape library systems, have been devised for the storage and handling of magnetic tape reels. More sophisticated systems which have been used in the past, commonly referred to as automated tape library systems, permit storage and automatic retrieval of data contained on tape reels such as the conventional IBM 3420 tape subsystems. As is well known, such tape subsystems provide a nominal data rate of 1.25 megabytes per second, a recording density of 6250 bytes per inch, and a storage capacity of 165 megabytes on 9-track, 1/2 inch iron oxide magnetic tape wound about 101/2 inch reels. The tape reels, with tape supported by conventional vacuum columns, are loaded upon a drive system including a laminated magnetic head.
One prior art automated tape library system utilized for storing and retrieving data contained on such tape reels is disclosed in U.S. Pat. No. 3,920,195. The apparatus taught therein corresponds generally to the Xytex Corporation XTL tape library which, under the control of an IBM operating system, automatically brings tapes from storage positions on either side of a linear rack, mounts them on tape drives, dismounts them when the job is completed, and returns each reel to storage. Average access time for delivery of a single tape reel to a selected drive in a medium size system (accommodating up to 2.3 trillion bits of storage in 3200 magnetic tape reels), however, is relatively slow. Such access times include selector-positioning mechanism access, reel selection, access to the automatic mounting unit, and the mounting of the reel on the tape drive (i.e., elapsed time from the operating system command to mount a tape to the time that the tape is physically mounted and ready to start the tape drive load cycle).
All reel storage matrices within the linear racks are serviced by the reel selector-positioning mechanism, which is part of the library control unit but which moves through each library storage unit on a connecting rail. The selector-positioning mechanism selects and replaces reels from the library storage units through use of a segment which rotates 180.degree. for the selection and replacement of reels on each side of the linear library storage unit.
When a reel mount command is received by the library control unit, the selector-positioning mechanism is commanded to move to the address of the required reel in the array. After the selector-positioning mechanism reaches the addressed position, the reel is drawn from its storage location and the selector-positioning mechanism is then moved to the automatic reel mounting unit which will service the particular tape drive that the system indicates. When the selector-positioning mechanism reaches that location the reel is deposited in a pre-load position. The automatic reel mounting unit then transports the reel to the tape drive and mounts it.
The automatic reel mounting unit retracts within the storage unit when not in use. There is a protective hood enclosing the drive hub of the tape drive which provides safety and security, and which can be pivoted out of the way by an operator to facilitate cleaning and servicing. Pivoting the hood automatically removes the drive from control of the system and makes it accessible for manual loading.
One particular drawback to the aforedescribed automatic tape library system, however, resides in its linear storage configuration. Like the IBM 3850 Mass Storage System, the system described in U.S. Pat. No. 3,920,195 suffers from certain access inefficiencies in cases where a selected tape reel is located at one end of the library, while the next selected tape reel is stored at the other end. Moreover, while the storage density per unit area of floor space taken up by the system is doubled through use of a two-sided linear rack, large storage systems incorporating more than one library storage unit merely aggravate the problems of linear storage by extending the path through which the selector-positioning mechanism must go to transit from one end of the library to the other.
Nevertheless, the use of standardized magnetic tape reels has been more recently supplanted by small, rectangular cartridges such as those which are used in the IBM 3480 tape subsystem. The rate at which data can be stored in the cartridge's one-half-inch wide chromium dioxide tape, or retrieved from it, is the result of using 18 recording tracks and achieving a linear data recording density of about 38 thousand bytes per inch (approximately six times the density used in typical tape reel drives). Moreover, the four-by-five inch cartridge used in the IBM 3480 tape subsystem is about one-fourth the size of a standard 10.5-inch reel of magnetic tape, yet it stores up to 20% more data, a total of 200 million characters. The tape and cartridge requirements for the system are as defined in the IBM document "Tape and Cartridge Requirements for the IBM 3480 Magnetic Tape Drives", GA 32-0048-0, the contents of which are incorporated herein by reference. Further details are disclosed in U.S. Pat. No. 4,426,047 and U.S. Pat. No. 4,383,660, and in the "Second Draft, Proposed American National Standard, Unrecorded, Magnetic Tape and Cartridge for Information Interchange" (ANSI-X 3 B 5/85-030, Feb. 1985), each of which is incorporated herein by reference.
While the advances provided by the digital servo control and new head technology of the IBM 3480 Magnetic Tape Subsystem permit low acceleration tape motion thereby eliminating the need for vacuum columns, capstans, and reflective markers have improved the level of data reliability as compared to conventional drives, the advantages gained are nearly offset by the disadvantages accruing from the use of book shelf type storage racks such as those described in U.S. Pat. No. 4,600,107. Furthermore, while the now-standardized IBM 3480 magnetic tape cartridges improve such data reliability through reduction of both contaminants and handling damage, the storage and retrieval of such cartridges must for the most part be carried out by human operators. Such human intervention not only degrades the reliability of selection and replacement, but also prolongs the time from which a data request is made and that data is read by loading a specific cartridge in an available tape drive. While recent improvements to systems utilizing the IBM 3480 cartridge which incorporate magazine type automatic loaders have reduced the time that jobs wait for cartridges to be mounted, such systems still require operator assistance and are inflexible as to the mounting order once installed within the magazine. It would, therefore, be desirable to provide a storage and retrieval system for magnetic tape cartridges such as those of the IBM 3480 type which are capable of storing large amounts of data, while consuming as little floor space as possible, and at the same time making the data readily available by minimizing human intervention.
Two such approaches used in the past are disclosed and claimed in U.S. Pat. No. 3,938,190 and U.S. Pat. No. 4,527,262. Unlike each of the above described storage and retrieval systems, the systems shown in U.S. Pat. No. 3,938,190 and U.S. Pat. No. 4,527,262 are both adapted for storing information-bearing units or modules in a polygonal configuration. For example, the storage and retrieval system for magnetic tape cassettes described in U.S. Pat. No. 3,938,190 includes a fixed two-dimensional storage array, a fixed processing or play station for extracting information from the units, and a movable selection mechanism having three degrees of freedom for retrieving individual modules from the storage area, transferring them to the processing area, and then returning them to the storage area after the information contained therein has been extracted. Additionally, the system includes means for automatically sequencing a series of units, as well as a preprocessing station for cuing up individual units for intra-unit accessing.
The information storer and retriever shown in U.S. Pat. No. 4,527,262, on the other hand, includes a plurality of shelves disposed so as to be substantially tangent to a cylinder of reference of a given diameter defining an access corridor which is large enough to accommodate a transport apparatus in the approximate center of the corridor. The transport mechanism includes a carriage that is translatably disposed on a pair of parallel rods to effect X-axis translation of the carriage through the access corridor of the polygonal arrangement, and means for rotating the transport mechanism about the X-axis to select a particular shelf disposed within the polygonal arrangement. Once the appropriate angular position is achieved and the carriage has been translated in the appropriate distance along the X-axis, then the in and out movement or X-axis translation is ready for activation by appropriate means.
Each of the aforedescribed U.S. Pat. Nos. 3,938,190 and 4,527,262 effectively stores and retrieves a plurality of information bearing media, but both are limited in the amounts of discrete media which may be contained therein. The substantially circular arrays provided increase the amounts of data which may be stored per unit floor space, but neither system includes the capability for expansion of storage by interconnecting individual storage units, one with the other, to improve data handling efficiency.